An OFDM technology is a high-speed multi-carrier transmission technology capable of effectively resisting Inter-Symbol Interference (ISI). Being different from a conventional frequency division multi-path transmission method, sub-carriers of an OFDM system are orthogonal to each other, so frequency spectra between sub-channels are allowed to be overlapped with each other, thereby maximizing the use of spectrum resources. The OFDM technology has been successfully applied to systems, such as an Asymmetric Digital Subscriber Line (ADSL), a Wireless Local Loop (WLL), a Digital Audio Broadcasting (DAB), a High-definition Television (HDTV), and a Wireless Local Area Network (WLAN). The application of the OFDM technology in the mobile communication is an irresistible trend.
Channel estimation is one of core technologies of the OFDM system. An existing channel estimation method of the OFDM system mainly includes linear interpolation channel estimation, Wiener filtering channel estimation, and Fast Fourier transform (FFT) interpolation channel estimation.
In terms of performance, the linear interpolation channel estimation is the worst; in terms of implementation complexity, the Wiener filtering channel estimation is more complex than the other two channel estimation methods, and has the largest operations; and the FFT channel estimation has the smaller operations and is relatively more simply implemented.
In the actual OFDM system, an OFDM symbol sent by a sending apparatus in a frequency band includes a plurality of sub-carriers bearing data and a plurality of pilot sub-carriers used for channel estimation. Frequency-domain sub-carriers of the OFDM system are usually placed at equal intervals. FIG. 1 is a distribution diagram of frequency-domain pilots obtained after a receiving apparatus performs channel estimation in time dimension on received signals. Assuming that a total number of sub-carriers of the OFDM symbol is Y, sequence numbers of the sub-carriers are [0, Y−1], an interval of adjacent pilot sub-carriers is P, and after the channel estimation in time dimension, a total number of the pilot sub-carriers in one OFDM symbol is N. The interval P of adjacent pilot sub-carriers is selected to guarantee that a frequency interval is less than channel coherent bandwidth, that is, to satisfy a sampling theorem, and in this way, an actual channel frequency response can be restored.
In fact, in the OFDM system, a bandwidth frequency band of each OFDM symbol also includes protection sidebands which are set zero. FIG. 2 is a schematic diagram of an OFDM symbol in a Long Time Evolution (LTE) system on bandwidth, in which the bandwidth occupied by the OFDM symbol includes 1024 sub-carriers. A total number of effective sub-carriers in the 1024 sub-carriers is 600. The 600 effective sub-carriers include sub-carriers bearing pilots and sub-carriers bearing data. The bandwidth occupied by the 600 effective sub-carriers may be 10 MHz. The 1024 sub-carriers of the OFDM symbol further include protection sidebands on both ends respectively occupying 212 sub-carriers and 211 sub-carriers. The 1024 sub-carriers of the OFDM symbol also include one Direct Current (DC) sub-carrier. When the FFT interpolation channel estimation is adopted, the channel estimation is performed according to information of the pilot sub-carriers in the 600 effective sub-carriers. The pilots just perform sampling on frequency bands occupied by the effective sub-carriers of the OFDM symbol instead of performing sampling on frequency bands occupied by the whole OFDM symbol including the protection sidebands. Therefore, a large error exists between the channel obtained through the FFT interpolation channel estimation method and the actual channel. The error is relatively smaller at an intermediate frequency point, but is larger at edge frequency points on the both ends, and for ease of description, the phenomenon is called an edge effect.
In the process of researching and practicing the prior art, the inventors of the present invention find that adoption of the FFT interpolation channel estimation method causes a large error of the channel estimation, and in particular, causes a large error at edge frequency points of bandwidth of an OFDM symbol.